1. Field of the Invention
The present invention relates to a photo detector having a plasmon focusing antenna, and to an optically interconnected LSI using such photo detectors.
2. Description of the Related Art
In recent years, the operating speed of large-scale integrated circuits (LSIs) has greatly increased, thanks to the improved performance of electronic devices such as bipolar transistors and field-effect transistors. The performance has been improved, owning to the decrease in transistor size. However, since the wires connecting transistors are now thin, the line resistance and the inter-line capacitance have increased, making great problems. These problems are a bottleneck of the improvement in LSI performance.
In view of the problem resulting from electric wires, some types of optically interconnected LSI, in which the components are interconnected by optical lines, have been proposed. Power loss in optical lines has virtually no dependence on frequency, ranging from direct current to 100 GHz or more. Further, the optical lines impose virtually no electromagnetic interference. Lines several 10 Gbps or more can therefore be realized easily.
The optical lines in an LSI need high-speed photo detectors made of silicon (Si), i.e., the substrate material of the LSI. Si is an indirect transition semiconductor, and its optical absorption efficiency is generally low. Inevitably it can hardly achieve both high light-receiving efficiency and high operating speed.
To solve this problem, photo detectors of plasmon focusing antenna type are known. Any element of this type utilizes surface plasmon that propagates in the surface of an electrically conductive member made of, for example, metal. (See Japanese Journal of Applied Physics, Vol. 44, No. 12, and p. L364 (2005), hereinafter referred to as Document 1.) Light focusing and light passage through a small opening, both achieved by surface plasmon, are known in the art. (See Optics Letters, Vol. 26, No. 24, and p. 1972 (2001), referred to as Document 2.) On the other hand, in laser elements that differ from the photo detectors of plasmon focusing antenna type, a small asymmetrical opening may be used to improve the efficiency of transmitting light through a small opening. (See JP-A 2001-189519 (KOKAI), referred to as Document 3).
In the technology disclosed in Document 1, photoelectric conversion must be performed immediately after the light focused by the plasmon focusing antenna passes through the small opening. That is, a light-receiving Si layer must be arranged at the exit of the small opening in order to accomplish the photoelectric conversion. If the small opening is long (the length is equivalent to the thickness of an electrically conductive thin film), the light will be greatly attenuated as it passes through the small opening. Therefore, the electrically conductive thin film constituting the plasmon focusing antenna must be thin, but not so thin to allow the passage of light.
Hence, to apply optical lines to LSIs, one of the following two methods should be used. In one method, photo detectors of plasmon focusing antenna type are formed in transistor-forming surface (i.e., surface of the SI substrate). In the other method, photo detectors of focusing antenna type are formed on the multilayer interconnection lines of the LSI, and signals these elements have generated through photoelectric conversion are transferred via electric lines.
However, the method, in which photo detectors of plasmon focusing antenna type are formed in a transistor-forming surface, will reduce the number of transistors than can be formed in unit area of the LSI. Since method impairs the essential function of the LSI, it is not practical. The method, in which photo detectors of focusing antenna type are formed on LSI multilayer interconnection lines and electric wiring from these elements the surface of the Si substrate, is likely to make problems, such as signal degradation due to parasitic LCR in the electric lines, an increase in noise, and crosstalk to the other electric lines. In other words, the optical lines may impair the high operating speed.
In the method disclosed in Document 2, it is desirable to make the opening smaller in order to achieve a high-speed response.
If the opening is made smaller, however, the amount of light that passes through the surface plasmon may decrease.